Different Concrete Prestressing Methods and System | Civil Engineering Optional Notes for UPSC PDF Download

Methods of Prestressing

Prestressing is a technique used in construction to enhance the performance of concrete structures by inducing internal stresses. This can be achieved through several methods, broadly classified into internal and external prestressing.

Internal Prestressing

Internal prestressing involves applying a prestressing force directly to the steel reinforcement within the concrete. This method induces compressive stresses in the concrete, which counteract tensile stresses that arise under load. There are two main methods of internal prestressing:

1. Pre-tensioning
2. Post-tensioning

Pre-tensioning Method:

• Process: Tendons are tensioned before the concrete is placed. The tendons are temporarily anchored against abutments and pulled using jacks. Concrete is then poured and allowed to harden. Once the concrete reaches sufficient strength, the tendons are released, transferring the stress from the steel to the concrete through bond.
• Usage: Commonly used for small-sized members such as beams, slabs, piles, sleepers, and electric poles.
• Advantages: Reliable and durable as it is performed in controlled factory conditions.
• Disadvantages: Limited to smaller sections; higher losses due to shrinkage and creep.

Post-tensioning Method:

• Process: Tendons are tensioned after the concrete has hardened. Ducts are formed within the concrete member using tubes or metal sheathing. After the concrete has set, tendons are tensioned and anchored with wedges. The space between the tendon and duct is filled with cement grout.
• Usage: Suitable for both precast and cast-in-situ constructions, especially for large span structures like bridges.
• Advantages: Can be performed on-site, lower prestress losses, suitable for large and heavily loaded structures.
• Disadvantages: More expensive due to the use of sheathing.

Comparison of Pre-tensioning and Post-tensioning

External Prestressing

External prestressing involves applying the prestressing force by adjusting external reactions through different support conditions. This method requires high accuracy in planning and execution and can use both pre-tensioning and post-tensioning techniques.

Prestressing Systems

1. Magnel Blaton System:

   • Uses a rectangular cable section with layers of wires.
   • Wires are separated and maintained in a geometric pattern using grills or spacers.
   • Minimal frictional resistance allows wire movement during tensioning.

2. Gifford Udall System:

   • Wires are stressed and anchored individually using Udall grips.
   • Each grip consists of two half cones, bearing against a thrust ring cast into the concrete.

3. P.S.C. Monowire System:

   • Wires are tensioned individually.
   • Anchorages use a collet sleeve wedging in a conical hole, with a central block to anchor central wires.

4. C.C.L. Standard System:

   • Individual wire tensioning.
   • Cables consist of multiple wires with spacers to reduce friction and ensure proper grouting.
   • Uses C.C.L. anchor grips for anchoring.

5. Lee-McCall System:

   • High tensile alloy steel bars are used as tendons.
   • Bars are anchored using threaded nuts bearing against a distribution plate.

6. Chemical Prestressing:

   • Uses expanding cement to induce prestress.
   • Concrete members are held at both ends by rigid supports, and expansion of the cement induces prestress.

Each of these methods and systems has specific applications, advantages, and limitations, making them suitable for different construction scenarios and requirements. By choosing the appropriate prestressing technique, engineers can ensure the structural integrity and longevity of concrete structures.